Magnetoelastic torque transducers rotating symmetrically and similar in design to the invention are known through a number of patent specifications and articles, some of which will be commented upon below.
The common measuring principle and common feature of these known solutions is that zones have been created in the magnetoelastic material with anistropy or magnetic orientation substantially coinciding with the compressive or tensile stresses appearing in a shaft, at .+-.45.degree. in relation to a generatrix to the shaft, when this is caused by the load to twist. In this way the reluctance in a zone whose direction of magnetization coincides with the tensile stress will be reduced and the reluctance in zones coinciding with the compressive stress will increase, thus causing a corresponding increase or decrease in the flow through the zones. This is applicable in the case of positive magnetostriction.
By measuring the difference in reluctance between two zones in which the anisotropy in one zone is directed along the direction of pull and that in the other zone along the direction of pressure, a measurement of the torque is obtained which has little sensitivity to axial forces of flexural stresses.
The difference in reluctance between the zones is usually measured by creating a time-dependent H-field via a primary coil concentric to the shaft, directed along the shaft and having equal amplitude in both zones. The difference in B-field between the zones is then measured using two identical secondary coils, one above each zone. The easiest way to do this is by inverseconnecting the secondary coils in such a way that the stresses induced in each coil are subtracted from each other. Phasesensitive rectification of the secondary signal obtained in this manner also enables torques of different direction to be distinguished.
The difference between the torque transducers mentioned below, besides the choice of magnetostrictive material, lies in the different methods of creating the required anisotropy.
According to Russian Pat. No. SU 667836 the anistropy is created by cutting slits in the surface of the shaft.
Russian Pat. No. SU 838448 describes a transducer in which an attempt is made to increase sensitivity instead by producing the slits by roll-embossing this pattern in the surface.
American U.S. Pat. No. 4,506,554 covers a transducer in which the anisotropy is achieved by using a sleeve with slit cut in the main stress directions.
A very similar design is also described in IEEE Trans Magn Vol Mag-22 No. 5 p. 403 (Sep. 1986) in an article entitled "Torque sensors using wire explosion magnetostrictive alloy layers" by J. Yamasaki, K. Mohri et al. Here a 100 micrometer thick "sleeve" has been produced, with through-"slits" on a stainless steel shaft by spraying the shaft with drops of a molten magnetoelastic alloy through a mask. The technique used consists of allowing a strong electrical discharge to pass through a conductor in the relevant material, whereupon the core is vaporized and the conductor explodes.
An article by K. Harada, I. Sasada et al in IEEE Trans Magn Vol Mag 18, No. 6, p. 1767 entitled "A new torque transducer using stress sensitive amorphous ribbons" describes a solution comprising glueing a stress-relieved foil of amorphous magnetostrictive material onto a shaft which has been prestressed with a certain torque. When the glue has dried, the prestressing will disappear and produce an anisotropy since the foil will now be prestressed.
In "Torque transducers with stress-sensitive amorphous ribbons of chevron-pattern", published in IEEE Trans Magn Vol Mag-20, No. 5, p. 951, I.Sasada, A.Hiroike, and K.Harada describe how anisotropy can instead be created by glueing strips of amorphous magnetoelastic material onto a magnetic or non-magnetic transducer shaft in its principle stress directions.
Offenlegungsschrift DE 3704049 A1 mentions a method of creating anisotropy by placing a conductor pattern on a shaft of magnetostrictive material. This conductor pattern has the same shape as the slits mentioned earlier.
Torque transducers of this type generally offer good measuring qualities for the majority of applications. However, in a few special applications problems of measuring accuracy may occur. If a thermal flux passes through the shaft, i.e. if there is a temperature gradient in the shaft, this will affect the measurement. It is known from the literature that the permeability of magnetic material is extremely temperature-dependent. If, therefore, a temperature difference exists across the measuring zones of the shaft, the measured signal will not correctly indicate the torque, due to the varying permeability within the measured zone. Applications in which these problems may occur can be found in many areas. Temperature differences of several hundred degrees may occur, for instance, between a combustion engine and its gearbox or coupling. A thermal flux will therefore pass through the shaft connecting these parts and the measuring accuracy will thus be affected when measuring torque in the shaft. An electric motor may be located in a well heated machine room and, due to high load, may reach a maximum temperature permitted for the motor. The machine being driven may be located outdoors and connected to the motor by a shaft passing through a wall and there is every likelihood of the shaft acquiring a temperature gradient which may cause problems in the measuring accuracy.
As far as we know, no information exists in the available literature, concerning the temperature-dependence of the permeability for the materials used in connection with magnetoelastic torque transducers of this type, i.e. both annealed and cold-rolled silicon steel. Some idea of expected magnitudes can, however, be deduced from a book published in 1951 by D van Nostrand Company Inc, Princetown, N.J., "Ferromagnetism" by Bozorth. This includes measurements applicable to pure iron which has been stress-relieved to 800.degree. C.
From this book, appendix 1, FIG. 3-8, it can be seen that both the initial permeability .mu..sub.0 and the maximum permeability .mu..sub.m increase as the temperature increases. It can be seen from the figures that .mu..sub.m increases by a factor of two at 200.degree. C., i.e. that EQU .mu.=.mu..sub.T=0 0.degree. C. (1+T/200)
or, in other words, that .mu. increases by 0.5%/.degree.C.
The magnetostrictive permeability alteration due to the tensile stress in a stress-relieved strip of the material used in the type of torque transducer under consideration has been measured and is at least 1%/MPa.
For a moderately loaded transducer where the material is loaded to 20 MPa, therefore, maximal loading would give the same permeability alteration as a temperature alteration of 40.degree. C. This would thus give a neutral drift of 1/40=2.5% of max. signal per .degree.C. temperature difference between the measured zones.
The above estimate is extremely rough and assumes, for instance, that the magnetostrictive permeability alteration for compressive stresses is substantially the same as that measured for tensile stresses. However, the estimated value should be representative for the maximum neutral drift caused by temperature gradients. This also shows that in applications where temperature gradients are likely in the shaft measuring zones, and where measuring must be rather accurate, measures must be taken to eliminate or greatly reduce the effect of the heat flux passing through the shaft.
The invention to be described now shows a device which reduces the sensitivity to temperature gradients in magnetoelastic torque transducers in an extremely reliable manner.